(CNN) -- The recently merged Continental and United Airlines canceled 24 flights Wednesday due to pilot sick calls, which occurred as United Continental Holdings is negotiating a collective bargaining agreement with the pilots, an airline spokeswoman said.

The airlines, which merged last year to become the world's largest carrier, said the sick calls began Tuesday, largely impacting the airports in New York and Newark, New Jersey, said airline spokeswoman Megan McCarthy.

The Air Line Pilots Association didn't have an immediate comment Wednesday, but last month, the association said much work remains before the merger could be considered a success. The association and the merged airlines are in contract negotiations.

Capt. Wendy Morse, chairman of the United Master Executive Council of the Air Line Pilots Association, made that remark as shareholders gathered for an annual United Airlines meeting.

"Until an agreement is reached, this merger cannot be considered anything close to a success," Morse said in a statement in June. "It's unfathomable that, more than one year after the announcement of the UAL/CAL merger, the company has not reached agreement with its pilots on a JCBA."

Morse said that without an agreement with pilots, "the company will never be able to achieve the full benefits of the merger."